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Full text of "Text Book Of Mechanical Engineering"

Electric Welding.                       '     327

7 Electric Welding.—This important process, first intro-
duced in 1885, has proved of great advantage in satisfactorily
uniting pieces unattachable by ordinary means. Among these
articles are boiler plates, which must be our apology for intro-
ducing the subject here. Wrought Iron, or in a less degree Mild
Steel, were the only materials previously weldable, and even then
the joint had but 70 per cent, of the strength of the solid material
—a serious matter with crane chains, where every link is welded.
Scale might form between the weld, the heating could not be
seen openly, and might neither be even nor thorough; objections
all absent in electric welding.

Electric energy consists of two factors—electromotive force
(or pressure) multiplied by the current (volts x ampbres). If
this energy pass through a good conductor, nothing is observable
in the latter; if a bad conductor be presented, the current will
not pass ; but an indifferent conductor will allow some of the
energy to pass, while the rest is converted into heat on account
of the resistance, the amount of heat energy produced being
equivalent to the electric energy destroyed. The metals we most
desire to weld are in the class of semi-conductors, and there is no
difficulty in raising their temperature to welding point by the
electric arc; but the heating effect of a current is independent
of the pressure or potential, depending only on the value of the
current, and it follows that the energy from the dynamo must be
transformed, so as to obtain a low voltage with a high amperage.
Every one knows the galvanic battery and induction coil, where a
current of low potential becomes one of high potential after
passing the coil, though at £ sacrifice of current value, the energy
remaining the same. Transformers serve the same purpose,
being similarly designed, and it depends which *side of the trans-
former we are on as to what amperage we obtain.

There are two processes employed in electric welding, the
' Thomson' and the (Benardos/ named after Professor Elihu
Thomson and M. Von Benardos respectively. The first con-
sists in using the pieces to be united as the poles, and the second
in using one of the pieces as the negative pole, while the positive
pole is supplied by a rod of carbon, held, in/ the hand in the
manner of a soldering bit. The electric energy is obtainable in